CVE-2018-20685 in Solaris
Summary
by MITRE
In OpenSSH 7.9, scp.c in the scp client allows remote SSH servers to bypass intended access restrictions via the filename of . or an empty filename. The impact is modifying the permissions of the target directory on the client side.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2018-20685 affects OpenSSH version 7.9 and resides within the scp.c component of the scp client implementation. This security flaw represents a significant access control bypass issue that allows remote SSH servers to manipulate client-side file permissions through crafted filename arguments. The vulnerability specifically manifests when the scp client encounters filenames consisting of a single period or empty filenames during file transfer operations. This represents a critical weakness in the client-side validation mechanisms that should prevent unauthorized modifications to the local filesystem.
The technical exploitation of this vulnerability occurs through the scp client's handling of special filenames that are typically used for directory navigation or reference purposes. When a remote SSH server sends a filename containing a single period or empty string, the scp client incorrectly interprets these inputs and executes permission modification operations on the target directory rather than on the intended file. This misinterpretation stems from insufficient input validation and improper handling of edge cases in the filename parsing logic. The vulnerability falls under the category of improper input validation and can be classified as CWE-20, which addresses improper input validation in software systems. The flaw demonstrates a clear breakdown in the principle of least privilege as the client-side system allows remote entities to modify directory permissions without proper authorization checks.
The operational impact of this vulnerability extends beyond simple permission changes and represents a serious threat to system integrity and security. Remote attackers can exploit this weakness to modify directory permissions on the client machine, potentially allowing them to escalate privileges or create persistent access mechanisms. The modification of target directory permissions can enable attackers to gain unauthorized access to sensitive files, create backdoor access points, or disrupt normal system operations. This vulnerability directly impacts the confidentiality, integrity, and availability of systems that rely on secure file transfer protocols. The attack vector is particularly concerning because it requires no authentication from the attacker beyond establishing an SSH connection, making it an attractive target for automated exploitation campaigns. The vulnerability aligns with ATT&CK technique T1021.004 which covers remote services and T1059.001 for command and scripting interpreter, as it allows for unauthorized system modification through legitimate protocols.
The remediation approach for this vulnerability involves updating to OpenSSH versions that contain the appropriate patches addressing the scp client filename handling logic. System administrators should immediately deploy the latest security updates from their respective distribution channels to ensure protection against this specific exploit. Additionally, organizations should implement network segmentation and access controls to limit the exposure of systems running scp clients to untrusted remote servers. Monitoring for unusual file permission changes and implementing file integrity monitoring solutions can provide early detection of exploitation attempts. The vulnerability demonstrates the importance of thorough input validation and proper edge case handling in security-critical components, particularly those involved in file system operations. Organizations should conduct regular security assessments of their SSH implementations and ensure that all components are kept current with the latest security patches to prevent similar vulnerabilities from being exploited in their environments.